The invention relates to a process for producing reciprocal adhesion at the interface between two contacting layers, whereof one layer is formed from condensation crosslinking polysiloxane and the other layer is formed from addition crosslinking polysiloxane.
Both the condensation crosslinked and the addition crosslinked silicones are in the present case based on oily polyorganosiloxanes and are reinforced to a greater or lesser extent with fillers, so that they have many uses in a consistency varying from the highly fluid to kneadable, particularly as an impression material for dental purposes. It is also possible to use one of the components as an oil without any filler admixture.
Polyorganosiloxanes which have been conventionally used for many years from the structural chemistry standpoint comprise polydiorganosiloxane chains containing at least two chemically reactive groups with varying spacings or only at the chain ends. In joint reaction with other reactants in the presence of catalysts, these groups lead to bridging between the molecular chains. If elastomer products are formed, this crosslinking is called vulcanization. Depending on the degree of crosslinking, vulcanizates are formed with a greater or lesser elasticity and strength.
There is a fundamental difference between the two vulcanization methods, i.e. the condensation crosslinking and addition crosslinking methods, which are performed at ambient temperature.
Condensation crosslinking is based on the reaction of alpha-omega-dihydroxy-polydiorganosiloxanes with organic silicates. Polydimethyl siloxanoles are mainly used as polyorganosiloxanes in producing elastomers for impressions. Although the corresponding phenyl derivatives are less frequently used, the invention can also be employed in connection therewith.
Methyl silicates, mainly in polymer form, or tetraethyl silicate are mainly used for condensation purposes during which alcohols are split off, whilst linking silicon atoms via oxygen bridges in a known reaction. Often mixtures of both silicates are used. In addition, glycol silicates are used for crosslinking and have special advantages.
Catalysts are added to the crosslinking agents to speed up the reaction. A large number of substances are given for this purpose in literature, but usually organo-tin compounds are used, e.g. tin acetate, tin octoate, or dibutyl tin dilaurate. The vulcanization time can vary from a few minutes to several hours as a function of the reactance and catalysts used.
As a result of condensation polymerization, the vulcanizates gradually shrink to a final state, varying their dimensions at different speeds for three reasons:
1. molecular compression
2. evaporation of the alcohols formed as by-products
3. thermal shrinkage, if the impression is taken at elevated temperature, e.g. when taking an impression in the mouth in the dental field.
As a function of the composition of the vulcanizates, the thermal shrinkage can be 0.1 to 0.3%, if the temperature difference is approximately 14%. It is therefore recommended that these special impressions are kept for at least 30 minutes to 2 hours under ambient climatic conditions before being filled with plaster.
The raw materials and adjuvants for the condensation crosslinking impressions are generally much more advantageous from the cost standpoint than addition crosslinking elastomers. However, the latter are superior to condensation crosslinking materials with regards to the dimensional stability and tensile strength. It is possible to considerably reduce the evaporation tendency of the alcohols contained in the vulcanizate by using suitable additives and/or crosslinking agents, so that the dimensional stability comes close to that of addition crosslinked vulcanizates. Another possibility for reducing shrinkage is to use materials with a limited elastic deformation and then adhering them by means of a strong adhesive to a rigid substrate, e.g. the impression spoon conventionally used in dentistry. Shrinkage is then significantly reduced.
In a practical case, 0.46% was measured with free shrinkage, but only 0.16% when using an adhesive lacquer on a suitable substrate using specification No. 19 of the American Dental Association.
According to the specification, the impression material used had a strain in compression of 1.5%. According to this specification, the quality was largely in accordance with type I of impression materials and was close to that of addition crosslinking materials.
Addition crosslinking RTV silicone elastomers have been used for many years, e.g. when the vulcanizate requires a better tensile strength or shrinkage is to be kept low or a higher Shore hardness A is required, when said characteristics cannot be achieved with comparable condensation crosslinking silicone elastomers. However, the corresponding raw materials are much more expensive.
The requisite reactants are vinyl polydimethyl siloxanes with at least two terminal vinyl groups in the molecule and hydrogen polydimethyl siloxanes. As a function of the molecular structure, the vinyl component or the hydrogen polyorganosiloxane can perform the crosslinking reaction. The hydrogen of the hydrogen component is always transferred to the vinyl groups, the additive bond between the two components being produced whilst dissolving the double bonds. The molecular chains are interlinked, leading to vulcanization.
This reaction is performed in the presence of a platinum catalyst, generally formed from hydrogen chloroplatinic acid or its alcohol or siloxane complexes. The effective quantity is between 5 and 500 ppm, the vinyl component always being added.
It is also known to use rhodium compounds or cobalt carbonyl and manganese carbonyl as catalysts for this purpose.
For dental impression purposes, obviously only platinum compounds are used. However, addition crosslinking systems have only been used for this in the last few years after overcoming serious difficulties (German Pat. No. 2,249,822).
Such platinum catalysts are extremely sensitive to a large number of catalysts poisons, of the type constantly occurring in the environment. Even a trace-wise contact between the platinum-containing materials and the organo-tin catalysts of the condensation crosslinking silicones has a disadvantageous effect. Possibly due to this sensitivity, it was for a long time impossible to give reliably accurate vulcanization times for addition crosslinking silicone systems, because they became constantly slower with the shelf life.
Due to careful processing and selection of the raw materials, the disadvantage of these products has been eliminated to such an extent that they can now be used for dental impressions, where high precision and short vulcanization times are required.
However, difficulties are still caused by the sensitivity of hydrogen polyorganosiloxanes to water. Hydrogen is evolved if there is a weak acid or alkaline reaction. In the presence of hydroxyl group-containing substances or moist calcium sulphate, gas bubbles are formed at the contact surface with the vulcanization products. These bubbles are transferred to the mould material, particularly dental plaster and the mould surface becomes porous, which is highly prejudicial to the further processing in the dental laboratory. Various proposals have been made for obviating this disadvantage of addition crosslinking silicones. Thus, DOS No. 2,926,495 proposes the use of palladium or its alloys, as well as other metals in finely divided form in silicone mixtures. The aforementioned metals absorb the hydrogen. Although the sought objective is achieved, the vulcanizate is naturally more expensive. In addition, these costly metals have to be admixed with the entire vulcanizate, although the desired effect is only required at the surface.